The present invention relates generally to an illumination system, and more specifically to an illumination optical system designed for microscopes, etc. and including an aspheric optical element.
Among optical systems for converting light emitted from a light source by a collector lens into substantially parallel light beams and transmitting the substantially parallel light beams to an objective by way of a relay lens, for instance, there is a vertical incident illumination optical system for microscopes. Kobhler illumination is widely applied to the vertical incident illumination optical system for microscopes, as generally set forth below. FIG. 1 is a general schematic of Kohler illumination. Typically, the vertical incident illumination optical system is made up of, in order from a light source side, a light source 1, a collector lens 2, a relay optical element 3, an aperture stop 4, a field stop 5, a relay optical element 6, a reflecting element 7 such as a half-silvered or dichroic mirror, and an objective 9. Light emitted from the light source 1 is converted by the collector lens 2 into substantially parallel light beams to project a light source image by the relay optical element 3 onto the position of the aperture stop 4. Then, the light source image is used as a secondary light source to project it by the relay optical element 5 onto the vicinity 8 of the pupil position of the objective 9, thereby illuminating an object surface 10 in a uniform and bright manner. The field stop 5 is projected onto the object surface 10 through the relay optical element 6 and the objective 9.
Reference is here to made to Lagrange-Helmholtz equation. EQU .phi..times..theta..gtoreq..PHI..times..THETA. (a)
where:
.phi. is the size of a light emitting portion of the light source, PA1 .theta. is the numerical aperture of the collector lens on the light source side, PA1 .PHI. is the range of illumination on the object surface, and PA1 .THETA. is the numerical aperture on an object surface side (limited by the numerical aperture of the objective). PA1 Z is coordinates for an optical axis direction, PA1 R is a center radius-of-curvature for a Y-direction, PA1 K is a conical coefficient, PA1 A.sub.4 is a fourth aspherical coefficient, PA1 A.sub.6 is a sixth aspherical coefficient, and PA1 A.sub.8 is an eighth aspherical coefficient.
From this equation, it is found that, to provide as uniform and bright illumination of viewing field as possible, the light emitting portion of the light source 1 should give out light uniformly, and the product of the size of the light emitting portion of the light source 1 and the numerical aperture of the collector lens 2 on the light source side should be greater than that of the numerical aperture of the objective 9 and the viewing range. To this end, it is required to use for the light source 1 a light source having as uniform and large a light emitting portion as possible and increase the numerical aperture of the collector lens 2. However, it is difficult to make the light emitting portion of the light source uniformly large. To obtain uniform and bright illumination, it is thus required to increase the numerical aperture of the collector lens 2 on the light source 1 side. The light source 1, for instance, a mercury vapor lamp, has such orientation characteristics as shown in FIG. 2, and so it is desired that the numerical aperture of the collector lens 2 on the light source 1 side be beyond the limits of orientation characteristics. For this purpose, an effort has been made to increase the numerical aperture of the collector lens 2 on the light source 1 side.
As well known in the art, the aspherical effect is effective to make correction for spherical aberration and the amount of deviation from the sine condition. In other words, it is possible to use one or more aspheric surfaces to make correction for these aberrations, thereby achieving an increase of numerical aperture. For instance, when light emitted from one point is converted to parallel light beams, it is known that the spherical aberration can be perfectly corrected by use of an aspheric single lens having a spherical surface on the light source side and a nearly paraboloidal surface on the parallel light beam side. However, a problem with this case is that a failure in satisfying the sine condition results in a sharp coma increase at an increased field angle for the light beams. This problem may be solved by correcting the amount of deviation from the sine condition as much as possible to achieve an increased numerical aperture while spherical aberration is produced to a certain extent, as disclosed in JP-B's 46-18781 and 46-18782.
In addition, it is known that an aplanatic lens not only free from spherical aberration but meeting the sine condition as well can be obtained by use of a single lens having an aspherical shape at each surface. For instance, JP-B's 46-18781 and 46-18782 disclose a high F-number lens corrected for spherical aberration and the amount of deviation from the sine condition by use of a single lens having an aspherical shape at each surface. Likewise, JP-A 6-214155 discloses a high numerical-aperture condenser lens.
It is also possible to achieve an increased numerical aperture by combining an aspheric lens with a spherical lens, not by use of an aspheric single lens. For instance, JP-A 6-118301 shows a collector lens made up of, in order from a light source side, a positive spherical lens having a strong power and a meniscus plastic lens having an aspheric surface on one side and a positive power, thereby achieving a high numerical aperture.
With a prior art collector lens composed singly of a spherical lens, however, it is required to achieve an increased numerical aperture by making the collector lens as close to the light source as possible or increasing the outer diameter of the lens. Since the light source is generally covered with a glass tube, however, it is impossible to make the first surface of the collector lens on the light source side closer to the light source than a certain level. Further, the collector lens is generally designed to largely bend light rays to nearly parallel light beams by the strong positive power of the lens located on the light source side and, hence, the edge of the lens becomes thin as the outer diameter of the lens increases; however, such a lens cannot possibly be fabricated. Only by use of a spherical lens, it is thus difficult to ensure the numerical aperture of a collector lens as far as the limits of orientation characteristics of a light source.
Another possible approach to obtaining an increased numerical aperture is to divide a lens having a large positive power, thereby distributing the power. However, this incurs a large increase in the number of lenses, resulting in added cost, and a transmittance drop as well. In other words, it is not easy to obtain a collector lens having high illumination efficiency.
Some examples of the aspheric single lens having one aspheric surface, disclosed in JP-B's 46-18781 and 46-18782, may be fully corrected for spherical aberration and the amount of deviation from the sine condition, even when they are used for the inventive illumination system to be described later. In this case, however, the numerical aperture achieved is not very high or 0.65 or less. Even if this technique is applied to the inventive illumination system, it is then unlikely that the left side of Lagrange-Helmholtz equation (a) has a value sufficient to satisfy the numerical aperture of the objective and illuminate the range needed for viewing.
If an aspheric single lens having an aspherical shape at each surface is used as disclosed in JP-A 6-214155 and JP-B's 46-18781 and 46-18782, it is prima facie possible to meet an aplanatic condition. However, it is only the vicinity of the optical axis which satisfies the aplanatic condition; in other words, coma occurs at positions off the vicinity of the optical axis. To achieve uniform yet bright illumination, it is also required to ensure the vignetting of off-axis light rays. However, an illumination system having large coma is no longer said to be a uniform yet bright illumination system.
JP-A 6-118301 shows a collector lens made up of a double-convex lens and an aspheric lens having one aspheric surface and a positive power. Generally when a collector lens is composed of a positive lens alone, spherical aberration is under-produced while coma is largely produced in the form of inside coma. Even when one aspheric surface is used in combination with the positive lens, it is thus difficult to ensure sufficient numerical aperture while the spherical aberration and coma are corrected at the same time.
Generally, a light source has a certain limited service life, and so light source replacement is required after the lapse of a certain period of use. For easy light source replacement, there should preferable be some distance (the working distance WD of the collector lens) between the center of a light source and the apex of a surface in the collector lens, said surface located nearest to the light source side. In the case of a single lens having a relatively short focal length, however, the position of the principal point cannot largely be moved even by means of bending. With the aspheric single lenses disclosed in JP-A 6-214155 and JP-B's 46-18781 and 46-18782, each having an aspheric surface at each surface, it is difficult to make the working distance of the collector lens wide while the focal length of the collector lens is kept constant.
With the techniques disclosed in JP-A 6-214155 and JP-B's 46-18781 and 46-18782, a high numerical-aperture collector lens may be achieved by use of one lens having an aspheric surface at each surface. However, this collector lens offers a fabrication problem because both surfaces are defined by aspheric surfaces, and much difficulty is encountered in increasing the precision of the aspheric surfaces as well. In the examples disclosed in these publications, light having a high numerical aperture is converted by one single lens to parallel light beams. Such an aspheric single lens having an aspheric shape at each surface, even when it is fabricated with a slight fabrication error, is likely to produce aberrations such as higher-order spherical aberration and coma, failing to keep the numerical aperture high. With a single lens having an aspheric shape at each surface, therefore, it is prime facie possible to design a collector having a high numerical aperture. However, it is actually very difficult to fabricate a collector lens having a high numerical aperture.
Unless the optical axes of both surfaces of a lens are in proper alignment with each other, then a decentered image is formed with aberrations such as spherical aberration. Consequently, a blurred image is projected onto a position off a place onto which a proper image is to be projected. In the case of the aspheric lens having an aspheric shape at each surface, disclosed in JP-A 6-214155 and JP-B's 46-18781 and 46-18782, it is very difficult in view of fabrication to keep the optical axes of both surfaces in proper alignment with each other. In addition, the power of the aspheric single lens having an aspheric shape at each surface is too strong to reduce the amount of decentration of the projected image and the amount of aberrations produced due to a misalignment between the optical axes of the aspheric surfaces. Even when the techniques disclosed in JP-A 6-214155 and JP-B's 46-18781 and 46-18782 are applied to the illumination system according to the present invention, it is thus unlikely that the light converted by the collector lens to substantially parallel light beams is precisely transmitted to the objective.